Solar Cell Characterization System With an Automated Continuous Neutral Density Filter

ABSTRACT

Techniques for solar cell electrical characterization are provided. In one aspect, a solar testing device is provided. The device includes a solar simulator; and a continuous neutral density filter in front of the solar simulator having regions of varying light attenuation levels ranging from transparent to opaque, the continuous neutral density filter having an area sufficiently large to filter all light generated by the solar simulator, and wherein a position of the continuous neutral density filter relative to the solar simulator is variable so as to control a light intensity produced by the device. A solar cell electrical characterization system and a method for performing a solar cell electrical characterization are also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 13/039,940filed on Mar. 3, 2011, the contents of which are incorporated byreference herein.

FIELD OF THE INVENTION

The present invention relates to solar cell technology and moreparticularly, to techniques for solar cell electrical characterization.

BACKGROUND OF THE INVENTION

Photovoltaic devices or solar cells have been recognized as one of theleading candidates of renewable energy sources capable of supplyinglarge scale clean energy that the world needs. The key metric in solarcell performance is power conversion efficiency that can be measuredfrom the J-V (current-voltage) characteristics under standard simulatedsolar radiation using a solar simulator system.

As part of an effort to push the power conversion efficiency in solarcells even higher, there is a need to perform a more comprehensiveelectrical characterization of the solar cell beyond basic “efficiency”measurements. Specifically, advanced characterization steps are needed,such as Dark-JV, Jsc-Voc (short circuit current—open circuit voltage)and Rseries (series resistance) Extraction (RsX), where one could obtainvarious parameters that reveal some information about junctioncharacteristics, parasitic (shunt and series) resistance and pseudo J-Vperformance. This information is very valuable for solar cell researchand development as this information provides deeper insight about whichaspects of the solar cell still need improvement.

The Jsc-Voc and RsX measurements require a variable light intensity.Intuitively, varying light intensity could be achieved with a solarsimulator by controlling a power level of the solar simulator, changingthe distance between the solar cell and the solar simulator, orinserting neutral density filters with different attenuation factors.But these methods cannot be performed easily and effectively. Namely,constantly switching the solar simulator power level in a large dynamicrange with a high rate will be very detrimental for the stability andlifetime of the lamp (typically a xenon lamp). Varying the distancewould require some sort of mechanical assembly and associated controlsto move the lamp up and down and thus would be impractical. Further, theresulting range of light intensities would be very limited and lightuniformity would vary a lot. Inserting various neutral density filtersis also unrealistic because too many filters will be needed, the processis difficult to automate and measurements cannot be performed at areasonable speed.

Recently, most of the Jsc-Voc measurement works are based on a flashlamp method. See, for example, U.S. Pat. No. 7,309,850 issued to R. A.Sinton et al., entitled “Measurement of current-voltage characteristiccurves of solar cells and solar modules” and R. A. Sinton et al., “Aquasi-steady-state open-circuit voltage method for solar cellcharacterization,” 16th European Photovoltaic Solar Energy Conference(May 1-5, 2000), Glasgow, UK, the contents of each of which areincorporated by reference herein. Using the flash lamp method,measurements can be performed rapidly by employing high speedelectronics. This method, however, suffers several drawbacks. Itrequires a separate system from the standard solar simulator system, themeasurement of Jsc is rather indirect since it actually measures thelight intensity and Jsc is assumed to be proportional with thisintensity, and the light source is not guaranteed to have the same solarspectrum throughout a large range of intensities. On top of thesedrawbacks, additional light source setup and requirements or high speedelectronics raise the cost of such a system, thus limiting itspopularity.

Therefore, improved variable light intensity measurement techniques foradvanced solar cell characterization, such as Jsc-Voc and RsXmeasurements, would be desirable.

SUMMARY OF THE INVENTION

The present invention provides techniques for solar cell electricalcharacterization. In one aspect of the invention, a solar testing deviceis provided. The device includes a solar simulator; and a continuousneutral density filter in front of the solar simulator having regions ofvarying light attenuation levels ranging from transparent to opaque, thecontinuous neutral density filter having an area sufficiently large tofilter all light generated by the solar simulator, and wherein aposition of the continuous neutral density filter relative to the solarsimulator is variable so as to control a light intensity produced by thedevice

In another aspect of the invention, a solar cell electricalcharacterization system is provided. The system includes a solar testingdevice having a solar simulator and a continuous neutral density filterhaving regions of varying light attenuation levels ranging fromtransparent to opaque, the continuous neutral density filter having anarea sufficiently large to filter all light generated by the solarsimulator; and a solar cell under test, wherein the continuous neutraldensity filter is located between the solar simulator and the solar cellunder test, and wherein a position of the continuous neutral densityfilter relative to the solar simulator and the solar cell under test isvariable so as to control a light intensity falling on the solar cellunder test.

In yet another aspect of the invention, a method for performing a solarcell electrical characterization is provided. The method includes thefollowing steps. A shutter of a solar simulator is opened to cause lightto be emitted from the solar simulator, the solar simulator being partof a solar cell characterization system having the solar simulator and acontinuous neutral density filter having regions of varying lightattenuation levels ranging from transparent to opaque, wherein thecontinuous neutral density filter is located between the solar simulatorand a solar cell under test. A transparent region of the continuousneutral density filter having no light attenuation is positioned betweenthe solar simulator and the solar cell under test. Light J-V data isobtained from the solar cell under test. The continuous neutral densityfilter is moved to position a region of higher light attenuation betweenthe solar simulator and the solar cell under test. Jsc-Voc data areobtained from the solar cell under test. The shutter is closed toprevent light from being emitted from the solar simulator. Dark J-V dataare obtained from the solar cell under test. The shutter is opened tocause light to be emitted from the solar simulator. The continuousneutral density filter is moved to position a plurality of regions oflower light attenuation between the solar simulator and the solar cellunder test. Three J-V curves are obtained from the solar cell under testto obtain RsX data for the solar cell under test.

A more complete understanding of the present invention, as well asfurther features and advantages of the present invention, will beobtained by reference to the following detailed description anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an exemplary solar cellcharacterization system according to an embodiment of the presentinvention;

FIG. 2 is a diagram illustrating an exemplary linear continuous neutraldensity filter (CNDF) mechanical actuator assembly that may be used inthe system of FIG. 1 according to an embodiment of the presentinvention;

FIG. 3 is a diagram illustrating an exemplary radial CNDF mechanicalactuator assembly that may be used in the system of FIG. 1 according toan embodiment of the present invention;

FIG. 4 is a diagram illustrating an exemplary graphical user interfacethat may be presented to a user of the system of FIG. 1 according to anembodiment of the present invention;

FIG. 5 is a diagram illustrating four steps needed for a comprehensiveelectrical (J-V) characterization of a solar cell according to anembodiment of the present invention;

FIG. 6 is a graph illustrating standard light J-V and pseudo J-V tracesof a solar cell with very high series resistance according to anembodiment of the present invention;

FIG. 7A is a diagram illustrating normal (standard) light J-Vmeasurement according to an embodiment of the present invention;

FIG. 7B is a diagram illustrating pseudo J-V measurement according to anembodiment of the present invention;

FIG. 8A is a diagram illustrating an exemplary linear type bandpassfilter containing a plurality of optical filters for quantum efficiencymeasurement according to an embodiment of the present invention;

FIG. 8B is a diagram illustrating an exemplary radial type bandpassfilter containing a plurality of optical filters for quantum efficiencymeasurement according to an embodiment of the present invention;

FIG. 9 is a diagram illustrating an optical bandpass filter being usedin conjunction with the system of FIG. 1 according to an embodiment ofthe present invention;

FIG. 10 is a graph illustrating quantum efficiency according to anembodiment of the present invention;

FIG. 11 is a diagram illustrating a first step in a solar cellelectrical characterization process using the system of FIG. 1 (a lightJ-V step) according to an embodiment of the present invention;

FIG. 12 is a diagram illustrating a second step in the characterizationprocess (a short circuit current versus open circuit voltage (Jsc-Voc)step) according to an embodiment of the present invention;

FIG. 13 is a diagram illustrating a third step in the characterizationprocess (a dark J-V step) according to an embodiment of the presentinvention;

FIG. 14 is a diagram illustrating a fourth step in the characterizationprocess (a Rseries Extraction (RsX) step) according to an embodiment ofthe present invention;

FIG. 15 is a diagram illustrating an exemplary report file according toan embodiment of the present invention;

FIG. 16 is an image of a linear-type CNDF according to an embodiment ofthe present invention;

FIG. 17 is an image of a radial-type CNDF according to an embodiment ofthe present invention;

FIG. 18 is a diagram illustrating an exemplary methodology forperforming a solar cell electrical characterization using the system ofFIG. 1 according to an embodiment of the present invention; and

FIG. 19 is a diagram illustrating an exemplary apparatus for performingone or more of the methodologies presented herein according to anembodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Provided herein is a solar cell characterization system having anautomated, large area continuous neutral density filter (CNDF)positioned between a solar simulator and a solar cell under test tocontrol the light intensity falling on the solar cell. FIG. 1, forexample, is a diagram illustrating exemplary solar cell characterizationsystem 100. Solar cell characterization system 100 includes a solartesting device and a solar cell under test. The solar testing deviceincludes a solar simulator 102, an automated CNDF 104, a motor 106, amotor controller 108, a J-V source meter 110 and a computer 112. Solarsimulator 102 and J-V source meter 110 are standard items in most solarcell laboratories. Thus, these components of the testing device are notdescribed further herein. An exemplary apparatus that may serve ascomputer 112 in system 100 is provided in FIG. 19 (described below).

Solar simulator 102 contains a lamp, e.g., a xenon, halogen or LED lamp,that will be used as a light source to create light 114 during the solarcell characterization steps (described below). The intensity of light114 produced by the lamp is governed by light source controller 116(e.g., a power supply) which is under the control of computer 112. Light114 can be turned ‘on’ or ‘off’ using a shutter on the solar simulator(not shown) which swings in or out blocking the light passage (thus thelamp itself is not switched on and off, but the shutter controls whetheror not the light produced by the lamp is emitted or not). The shuttercan also be under the control of light source controller 116. During thesolar cell characterization steps, source meter 110 takes J-V data fromthe solar cell under test while the software controls motor 106 throughmotor controller 108. See description below.

A neutral density filter is a filter that provides constant attenuationfor all different wavelengths. A CNDF has a range of light attenuationlevels (or optical density) from totally transparent to opaque. A blankarea where no filter is present is used to allow the total transparencyor 100% light transmission (see FIGS. 2 and 3, described below). A CNDFcould be fabricated by depositing a thin metal layer with varyingthickness or paint-brush spray on glass to achieve different lightattenuation. Another simple way to fabricate a CNDF is to print a smoothgradation of black to white image on a transparency sheet using an inkjet printer as shown in FIGS. 16 and 17, described below. The CNDFs(both linear and radial) presented for use herein are large areafilters. The term “large area” here means that the area of the filter islarger than the solar simulator's illumination area or illuminated area(see, for example, FIG. 11, described below) which is typically around 8centimeters (cm)×8 cm. This illumination area is much larger than thesolar cell area. What is important is the light level on the solar cellunder test can be controlled using the CNDF. In contrast, commerciallyavailable CNDFs are only available in small sizes, e.g., 2.5 cm×10 cm.

Two types of CNDFs can be used in accordance with the presenttechniques, a linear CNDF and a radial CNDF. CNDF 104 in FIG. 1generically represents either a linear or radial CNDF. Detaileddepictions of linear and radial CNDF filters are shown in FIGS. 2 and 3,respectively. The linear type CNDF is more suitable for large area solarcells but requires more complicated mechanical assembly, more componentsand has a larger footprint. The radial type CNDF is only suitable forsmall area solar cells but has fewer components, a simpler mechanicalassembly and a smaller footprint.

System 100 uses a mechanical actuator assembly which consists of asupport structure (not shown) and a motor, i.e., motor 106, that providesupport and motion to the CNDF filter. Motor controller 108, controlledfrom computer 112, provides the drive signal to motor 106.

A software system, e.g., loaded onto computer 112, provides control overthe measurement process. The software system communicates with sourcemeter 110 (to read the electrical J-V (current-voltage) data from thesolar cell under test), solar simulator 102 and motor controller 108.Another abbreviation for current-voltage is I-V which has the samemeaning as J-V. The software has a curve-fitting routine to performparameter extraction of all J-V data. Curve-fitting and parameterextraction for solar cell J-V curves are techniques known to those ofskill in the art and thus are not described further herein. The softwareis also programmed with additional functions useful for solar celltesting such as solar simulator light stabilization and solar celllight-soaking test. The light stabilization function utilizes a lightintensity sensor (not shown), typically a silicon photodetector, tomonitor the light intensity of the solar simulator. Any deviation fromthe target intensity (typically 1 sun) will be corrected by the computerprogram by adjusting the power of the solar simulator via the lightsource controller. The solar cell light-soaking test can be achieved byprogramming a certain sequence of solar cell testing using the computer.For example, the solar cell can be subjected to hours of light soakingwhile the cell performance is monitored periodically at a certain timeinterval (by way of example only, every 10 minutes). A user of system100 can save and load the measurement data and can also make a summaryof the device parameters that have been measured. An exemplary graphicaluser interface of the software is shown in FIG. 4, described below.

As highlighted above, there are two types of CNDFs that can be used insystem 100, a linear CNDF and a radial CNDF. FIG. 2 is a diagramillustrating exemplary linear CNDF mechanical actuator assembly 200.Linear CNDF mechanical actuator assembly 200 includes a chassis 202, afilter frame 204, a linear CNDF 206 mounted to filter frame 204 (using,e.g., double-sided tape or glue), rollers 208 mounted to chassis 202,stepper motor 210 and gear train 212 connecting stepper motor 210 torollers 208. A stepper motor is a motor capable of turning in a seriesof tiny but precise angular steps and is controlled using a set ofdigital power signals. Thus a precise amount of rotation can be achievedeasily.

During operation, a solar cell under test is positioned beneath theassembly, i.e., beneath CNDF 206, as shown in FIG. 2. Stepper motor 210then actuates filter frame 204/CNDF 206 relative to chassis 202 by wayof rollers 208. Namely, stepper motor 210 turns one or more of rollers208 which are in contact with and roll along a (top or bottom) surfaceof filter frame 204. According to an exemplary embodiment, stepper motor210 drives only the roller 208 that is directly connected to the motorthrough the gears. The other rollers 208 are passive rollers as theydon't drive the filter but only allow the filter to move freely. By thisaction, filter frame 204/CNDF 206 are actuated side to side (e.g.,left-to-right and right-to-left in FIG. 2) depending on whether steppermotor 210 turns clockwise or counter clockwise. Since the speed at whichthe motor turns is likely not the same as the desired speed for therollers, gear train 212 serves to coordinate the speed of the motor withthe desired speed of the rollers, e.g., through the use of reductiongears. This use of reduction gears would be apparent to one of skill inthe art and thus is not described further herein. As a result of theside to side movement, different areas (with different attenuationfactors, i.e., ranging from a blank area (totally transparent) to anopaque area (that does not let light pass through)) of the CNDF can bepassed in front of the solar simulator/light source (see FIG. 1), i.e.,between the solar simulator/light source and the solar cell under testboth of which are stationary. Since the CNDF is positioned between thesolar simulator/light source and the solar cell under test, the CNDFcontrols the light intensity falling on the solar cell. In other words,a position of the CNDF (relative to the solar simulator/light source andthe solar cell) can be varied so as to control a light intensityproduced by the device. Further, the present solar cell characterizationsystem allows variable and continuous intensity control of the solarsimulator while maintaining the same solar spectrum, which means thatthe neutral density filter imposes the same amount of attenuation acrossall wavelengths. Thus, the relative shape of the light spectrum remainsthe same, and only the amplitude of this intensity spectrum gets scaleddown.

FIG. 3 is a diagram illustrating exemplary radial CNDF mechanicalactuator assembly 300. Radial CNDF mechanical actuator assembly 300includes a filter frame 302, a radial CNDF 304 mounted to filter frame302 and stepper motor 306. As shown in FIG. 3, motor 306 is attached tofilter frame 302 by way of a gear assembly 310. Actuation of filterframe 302/CNDF 304 occurs by stepper motor turning in a clockwise orcounterclockwise manner, thereby turning filter frame 302/CNDF 304 inthe same clockwise or counterclockwise direction (see FIG. 3). As aresult of the rotational movement, different areas (with differentattenuation factors, i.e., ranging from a blank area (totallytransparent) to an opaque area (that does not let light pass through))of the CNDF filter can be passed in front of the solar simulator/lightsource (see FIG. 1), i.e., between the solar simulator/light source andthe solar cell under test both of which are stationary. Since the CNDFis positioned between the solar simulator/light source and the solarcell under test, the CNDF controls the light intensity falling on thesolar cell. In other words, a position of the CNDF (relative to thesolar simulator/light source and the solar cell) can be varied so as tocontrol a light intensity produced by the device.

CNDF 304 is depicted in FIG. 3 as having a radial shape (radiating froma common center), in this case a semi-circular (i.e., half-circle)shape, however, this configuration is merely exemplary. CNDF 304 mayhave any suitable semi-circular or complete (full) circle shape asdesired. The half-circle shape is more compact (smaller footprint) thanfor example a full circle radial filter. However, the “dynamic range” ofattenuation is maintained to be the same in both cases (full orsemi-circular), e.g., light transmission is changed from 1 to 10⁻⁴. Ashighlighted above, a radial-type CNDF (such as that shown in FIG. 3) canalso be used in system 100 of FIG. 1 besides the linear-type CNDF asshown in FIG. 1.

FIG. 4 is a diagram illustrating exemplary graphical user interface(GUI) 400 that may be presented to a user of system 100 (of FIG. 1), forexample, on a video display of computer 112 (see below), which wouldallow the user to perform a comprehensive solar cell J-Vcharacterization. As shown in FIG. 4, interface 400 includes variouspanels to control the measurement and to display the results of theparameter extraction. Interface 400 allows users to perform a four stepdevice characterization and analysis (as is described below). Interface400 is divided into panels according to their functions. The “Operation”panel allows the user to choose the measurement. The “Mode” panel allowsthe user to choose between measurement or data-viewing mode. The“Voltage Sweep” panel is for setting the J-V sweep range. The “System”panel is to control the solar simulator light. The “User profile” panelis to store and retrieve system settings belonging to certain users. The“Cell” panel is to specify basic parameters of the cell such as area andtemperature. The “Light J-V,” “Dark J-V,” “Jsc-Voc” and “Rseriesextraction” panels are to display the measurement results. The “Datafile” panel is to store or retrieve measurement results.

As highlighted above, in order to perform a complete, comprehensiveelectrical characterization of a solar cell (which is needed to increasethe power conversion efficiency), advanced characterization steps inaddition to basic efficiency measurements are needed. FIG. 5 is adiagram illustrating four steps needed for a comprehensive electrical(J-V) characterization of a solar cell, namely, light J-V which is abasic characterization step, dark J-V, Jsc-Voc (short circuit currentvs. open circuit voltage) and Rseries (series resistance) Extraction(RsX) which are advanced characterization steps. The Jsc-Voc and RsXsteps require variable light intensity, an issue that is solved by thepresent techniques. As shown in FIG. 5, light J-V characterizes overalldevice performance, dark J-V diode characteristics and parasiticresistance, Jsc-Voc characterizes pseudo-device performance and junctioncharacteristics, and RsX characterizes series resistance. Diodecharacteristics (also commonly termed junction characteristics) refer tothe diode behavior of the solar cell that can be described using thestandard diode equation:

J=J ₀*[exp(V/n*V _(T))−1],

wherein J₀ is the reverse saturation current, n is the diode idealityfactor and V_(T)=k_(B) T/q is the thermal voltage where k_(B), T and qare the Boltzmann constant, temperature and electron's charge,respectively. Parasitic resistance in solar cells are shunt (or leakage)resistance and series resistance. The shunt resistance (Rp) shorts thecurrent and the series resistance (Rs) dissipates the output power. Theseries resistance in the solar cell mainly arises from the top andbottom metal contact of solar cells and resistance of the “emitter” (toplayer) layer of the solar cell.

A pseudo J-V curve of a solar cell is essentially a shifted Jsc-Voc(short circuit current vs open circuit voltage) curve that reflects theJ-V characteristics of the solar cell if the series resistance isabsent. Both pseudo J-V and standard light J-V curves are shown in FIG.6. FIG. 6 is a graph 600 illustrating standard light J-V and pseudo J-Vtraces of a solar cell with very high series resistance (e.g., greaterthan about 10 ohms per square centimeter (ohm-cm²)). In graph 600,voltage V (measured in volts (V)) is plotted on the x-axis and current J(measured in milliamps per square centimeter (mA/cm²) is plotted on they-axis.

In standard (light J-V) measurement, one sweeps the solar cell with avoltage source. See FIG. 7A. In Jsc-Voc measurement, one essentiallysweeps the current source (from photo generated current) within thesolar cell by controlling the light intensity. See FIG. 7B. FIGS. 7A-Bare diagrams illustrating normal (standard) light J-V measurement versuspseudo J-V measurement. An intrinsic cell is the device components ofthe solar cells without the series resistance (Rs). Rs is considered anexternal factor due, e.g., to series resistance in the metal contactgrid. Thus it is often useful to exclude this Rs effect by measuringpseudo J-V measurements.

The short circuit current (Jsc) and open circuit voltage (Voc) aremeasured at terminals of the solar cell. For example, when the opencircuit voltage (Voc) is measured using a voltmeter, the voltmeterpractically draws no current. Thus the series resistance (Rs) haspractically no effect in the measurement (see FIG. 7B). Similarly, whenthe short circuit current is measured, all current flows to the ampmeterregardless of what the series resistance is. As a result, the Jsc-Vocpoints measured reflect the intrinsic J-V characteristics that is freefrom series resistance effects. However if the series resistance is veryhigh (e.g., greater than about 10 ohm-cm²) this approximation will breakdown because the voltage drop across the series resistance becomes verylarge compared to that of the solar cell (thus the J-V characteristicsare dominated by the series resistance and not by the solar cell).

From the Jsc-Voc curves one could extract the diode parameters: reversesaturation current (J₀₁) and diode ideality factor (n₁) more accuratelysince the Jsc-Voc measurement is free from series resistance effect thatwill otherwise bend the J-V curve down at high voltage. If the Jsc-Voccurve is shifted down by Jsc at 1 sun illumination (see, for example,FIG. 6, described above), the Jsc-Voc curve will meet the normal LightJ-V curve at the Jsc and Voc points. This shifted Jsc-Voc is also calledpseudo Light J-V. From the pseudo Light J-V, one can measure the pseudoefficiency (PEff) and pseudo fill factor (PFF) values that represent the“best maximum” efficiency of the cell if the series resistance isabsent. The pseudo efficiency and pseudo fill factor calculations areexactly the same as that of the “standard” efficiency and fill factor.This pseudo efficiency (PEff) and pseudo fill factor (PFF) informationis very useful as it reveals the maximum efficiency the cell couldpotentially have, and helps to distinguish whether a certain cell has aproblem of series resistance or an intrinsic device problem. Forexample, if the efficiency is much lower than the pseudo efficiency thatmeans that the power loss due to series resistance is very severe andthe cell intrinsically could perform much better if this seriesresistance is minimized

Besides performing a four-step J-V characterization as described above(i.e., light J-V, dark J-V, Jsc-Voc and RsX), as an additional optionthe present variable light intensity mechanical assembly and softwarecan advantageously be used to implement a fifth measurement function,i.e., a simple quantum efficiency spectrum measurement. Another filtercontaining a number of optical bandpass filters covering the wavelengthrange of interest for the solar cell which is typically from 300nanometers (rim) to 1400 nm would be needed. This filter can be a lineartype filter as shown in FIG. 8A or a radial type filter as shown in FIG.8B. Linear bandpass filter 802 (FIG. 8A) and radial bandpass filter 806(FIG. 8B) each contain a plurality of optical filters 804 and 808,respectively, for quantum efficiency measurement. As shown in FIGS. 8Aand 8B, each optical filter 804 and 808, respectively, covers adifferent wavelength λ. In the example shown in FIG. 8A, there are 7filters shown. In the example shown in FIG. 8B, there are 8 filtersshown.

According to an exemplary embodiment, solar cell characterization system100 (see above) having a CNDF would be used to perform the four-step J-Vcharacterization as described above (i.e., light J-V, dark J-V, Jsc-Vocand RsX). The CNDF would then be replaced with the optical bandpassfilter (e.g., linear bandpass filter 802 of FIG. 8A or radial bandpassfilter 806 of FIG. 8B) and quantum efficiency measurements wouldadditionally be performed. In one example, the optical bandpass filteris configured to have the same physical dimensions as the CNDF so thatthe two filters can be swapped into/out of system 100 with ease. By wayof example only, when a linear CNDF and a linear optical bandpass filter(such as that shown in FIG. 8A) are used, each filter could be the samesize and could be attached to filter frame 204 (see, FIG. 2, describedabove) in the same manner.

In this example where a bandpass filter is used in system 100 (of FIG.1), as the bandpass filter moves the solar cell under test will receivea monochromatic light set by the bandpass filter passing above it. Thesystem will measure the photocurrent (the short circuit current) of thesolar cell. Then by comparing the measured photocurrent with a similarmeasurement on a reference solar cell with known quantum efficiency(i.e., measurement on a reference solar cell done separately in place ofthe cell under test), one can calculate the external quantum efficiencyof the cell under test. The quantum efficiency of the cell under test isgiven by: QE_(X)=QE_(Ref)×I_(X)/I_(Ref), wherein QE is the quantumefficiency, J is the photocurrent and X and Ref subscripts refer to thecell under test and the reference cell, respectively.

FIG. 9 is a diagram illustrating an optical bandpass filter 902 beingused in conjunction with solar cell characterization system 100 (of FIG.1). It is notable that for ease of depiction, only a portion of system100 is shown in FIG. 9, however, the remainder of the system is the sameas that shown in FIG. 1 and described above. During operation, opticalbandpass filter 902 passes above the solar cell. The actuation of thefilter was described above, and that description is incorporated byreference herein. As also described above, the solar cell under test andsolar simulator 102 remain stationary, while filter 902 moves.

In practice, one could only have a limited number of bandpass filters asthe filter length is finite. Therefore only a limited number of datapoints can be obtained. Nevertheless, since practically all know quantumefficiency curves of solar cells are smoothly varying (no fastfluctuation involved), this method will yield a sufficiently goodapproximation to the real quantum efficiency curve. Considering adedicated quantum efficiency system is very expensive (e.g., about50,000 dollars and above as of the year 2010), this extra option bringsa high value-added functionality to the present solar cellcharacterization system. By comparison, in the case of variable lightintensity (using the variable light filters described above), one canhave any arbitrary amount of light attenuation by controlling theposition of the linear or radial light filter. Thus the resolution is“analog.”

FIG. 10 is a graph 1002 illustrating quantum efficiency. In graph 1002,wavelength λ is plotted on the x-axis and quantum efficiency or Q.E. isplotted on the y-axis. Graph 1002 illustrates the resulting quantumefficiency data obtained from measurements using the set-up of FIG. 9.

An exemplary method of operating solar cell characterization system 100will now be presented. FIGS. 11-14 are diagrams illustrating anexemplary methodology for performing a four step comprehensive solarcell electrical (J-V) characterization using system 100. As highlightedabove, two types of CNDFs can be used in accordance with system 100, alinear CNDF and a radial CNDF. In FIGS. 11-14, the actions of both alinear CNDF and a radial CNDF are shown along with the correspondingcharacterization step. It is notable that the linear CNDF depicted hasthe same configuration as shown in FIG. 2 with a blank area on the leftand which gets progressively darker moving from left to right.Similarly, the radial CNDF depicted has the same configuration as shownin FIG. 3 with a blank area on one end and which gets progressivelydarker as the filter is rotated (in this case counterclockwise).

The first step in the characterization process is a light J-V step. Inthis step, a light shutter (not shown) of solar simulator 102 is openedand filter 104 (linear CNDF or radial CNDF) is in a blank (no filter, noattenuation) position. See FIG. 11. Source meter 110 (see FIG. 1) takesthe J-V data by sweeping the voltage and measuring the current. Thesoftware reads this data and calculates all necessary device parameterssuch as efficiency (Eff), fill factor (FF), short circuit current (Jsc)and open circuit voltage (Voc).

FIG. 11 also illustrates that, as highlighted above, the area of thefilter is larger than the solar simulator's illuminated area. Accordingto an exemplary embodiment, the linear CNDF (the filter itself,excluding the frame) has a length l of from about 25 cm to about 75 cmand a width w of from about 10 cm to about 30 cm. The resulting area ofthe linear CNDF, given these exemplary dimensions, would be from about300 square centimeters (cm²) to about 1,875 cm². The length of theilluminated area, e.g., 12 cm, is shown for reference. According to anexemplary embodiment, the radial CNDF (the filter itself, excluding theframe) has a diameter d of from about 15 cm to about 40 cm. Theresulting area of the radial CNDF, given these exemplary dimensions,would be from about 350 cm² to about 2,500 cm². The angle of theilluminated area, e.g., 30 degrees, is shown for reference.

The second step in the characterization process is a Jsc-Voc step. Inthis step, CNDF 104 moves slowly towards a darker region (higher lightattenuation) (this movement from no attenuation to high attenuation cantake, e.g., about 30 seconds) and source meter 110 repetitively measuresshort-circuit current (Jsc) and open-circuit voltage (Voc) rapidly toobtain Jsc-Voc data points while CNDF 104 is moving. In the case of thelinear CNDF, CNDF 104 moves to the left to attain higher lightattenuation. In the case of the radial CNDF, CNDF 104 rotatescounterclockwise to attain higher light attenuation. See FIG. 12. Sourcemeter 110 takes the measurements concurrently as CNDF 104 is movingslowly. This results in a faster measurement time. Namely, over thecourse of the filter movement, source meter 110 measures the Jsc-Vocdata. According to an exemplary embodiment, source meter 110 takes aboutone data point each second. Each data point consists of a pair of Jscand Voc measurements that were taken rapidly one after another withinless than 0.25 seconds. CNDF 104 is moved by motor 106 by way of a motorcontroller 108 which is under the control of computer 112.Alternatively, it is also possible to have a user of the system move theCNDF manually by hand (i.e., without the use of an automated motor drivesystem).

The software then extracts all necessary device parameters such aspseudo-efficiency (PEff) and pseudo fill factor (PFF). The diodeideality factors (n₁ and n₂) and reverse saturation current (J₀₁ andJ₀₂) can also be extracted more accurately (as compared to Dark J-Vdata) since this data is free from series resistance effect (see above).The parameter extraction is based on a curve fitting of the J-V datawith a standard one- or two-diode model. A one-diode model is a solarcell model containing only one diode (with parameters J₀₁ and n₁). Atwo-diode model has a second diode (with parameters J₀₂ and n₂). Fromthe Light J-V and Jsc-Voc data one could also calculate the seriesresistance of the solar cell under test which provides an alternativemethod to the series resistance extraction in the RsX step, the fourthstep, described below. In this alternative method the series resistancecan be calculated as the difference of the voltage at the maximum powerpoint (MPP) of the Light J-V and Jsc-Voc data divided by the current atthat MPP point.

The third step in the characterization process is a Dark J-V step. Inthis step, the light shutter of solar simulator 102 is closed, i.e.,solar simulator 102 is not emitting any light. See FIG. 13. Source meter110 takes the J-V data again and the software reads this data andextracts all necessary cell parameters, such as diode ideality factors(n₁ and n₂), reverse saturation current (J₀₁ and J₀₂), shunt resistance(Rshunt) and series resistance (Rseries). As above, the parameterextraction is based on a curve fitting of the J-V data with a standardone- or two-diode model.

The fourth step in the characterization process is a RsX step. In thisstep, the light shutter of solar simulator 102 is again open. CNDF 104moves back (to the right in the case of the linear CNDF or clockwise inthe case of the radial CNDF) towards the lighter region (lower lightattenuation) and the blank region (e.g., under the drive of motor 106).CNDF 104 stops moving at three positions as shown in FIG. 14 (one ofthem is a blank region) where source meter 110 takes the J-V data. Thethree positions correspond to three light intensity levels (which arebased on the level of attenuation by the filter). One position should beat 100 percent (%) light intensity level (no attenuation) and the otherpositions are two slightly lower light intensity levels, such as 80% and60% (slightly higher attenuation). The series resistance determinedusing RsX measurement's result is insensitive to the light intensitylevels, for example one could choose 100%, 90%, 80% or 100%, 80%, 60%.The intensity level is proportional to the short circuit currentproduced by the solar cell under test. The software then calculates theseries resistance of the solar cell under test based on the three J-Vcurves using a “variable light-intensity method.” See, for example, M.Wolf and H. Rauschenbach, “Series resistance effects on solar cellmeasurements,” Adv. Energy Cony., vol. 3, pp. 455-479 (1963)(hereinafter “Wolf”), the contents of which are incorporated byreference herein.

According to an exemplary embodiment, the light intensity in this fourthstep is changed to three different levels and the J-V of the solar cellis measured at each intensity level. The three light intensity levelsare chosen to be one at 100% and two others at slightly lowerintensities such as approximately 80% and 60%. The series resistancecould be calculated following the method described in Wolf. First, themaximum power point (call it point A) of the first curve (usually under1 sun illumination) is chosen as the first intensity level and then asecond point B is identified on the second J-V curve by going down byΔIsc from A, where ΔJsc is the difference of the short circuit currentbetween the first and second curve. The same procedure is repeated toidentify point C on the third J-V curve. The 1/slope of linear fitthrough point A, B and C will yield the series resistance.

As highlighted above, the solar cell electrical (J-V) characterizationprocess using system 100 can be completely automated. In that exemplaryconfiguration, the steps of the process are run and coordinated bycomputer 112. This completely automated methodology is further describedin conjunction with the description of FIG. 18, below.

A user of system 100 does not necessarily have to perform all of thesefour operations at once. System 100 and its associated software allowsthe user to choose any combination of the four characterization steps toperform, or all of the steps. From these measurements, the user couldobtain many parameters of the solar cells at once. A very good solarcell should have the following parameters to be as large as possible:efficiency, fill factor (also the pseudo efficiency and pseudo fillfactor) and shunt resistance (Rshunt), but the following parameters haveto be as small as possible: series resistance, reverse saturationcurrent (J₀) and diode ideality factor (n₁ and n₂). The subscripts 1 and2 refer to diode number 1 and 2, respectively in the two-diode model ofa solar cell.

Based on the measurements taken, the software can also generate a reportfile as shown in FIG. 15. FIG. 15 is a diagram illustrating exemplaryreport file 1500 from a measurement session. Report file 1500 summarizesplots and various solar cell parameters derived from the four step J-Vcharacterization (Light JV, dark JV, Jsc-Voc and RsX). By performingthis four step J-V characterization, system 100 serves as a veryvaluable and efficient tool for solar research and development.

FIG. 16 is an image of a linear-type CNDF. The CNDF shown in FIG. 16 wasmade by ink-jet printing a gradation of black to white pattern on atransparency film and mounting the film onto a frame. The frame shouldbe black in color to block the light outside the filter area. FIG. 17 isan image of a radial-type CNDF. The CNDF shown in FIG. 17 was made inthe same manner as the CNDF of FIG. 16. Specifically, the radial CNDFwas made by ink-jet printing a gradation of black to white pattern on atransparency film and mounting the film onto a black frame.

As highlighted above, the present solar cell electrical (J-V)characterization process using solar cell characterization system 100can be completely automated, i.e., run and coordinated by computer 112.By way of example only, FIG. 18 is a diagram illustrating exemplaryautomated methodology 1800 for performing a solar cell electricalcharacterization using system 100. In step 1802, a light source of solarsimulator 102 is turned on. According to an exemplary embodiment, thisis accomplished by opening a shutter on solar simulator 102, which asdescribed above can be under the control of computer 112.

In step 1804, CNDF 104 is positioned in a completely transparent (blank,no attenuation) position. As described above, the present CNDFs have ablank area and the movement of the CNDFs can be coordinated by computer112 (e.g., by way of motor 106 and motor control 108).

In step 1806, light J-V data is obtained from the solar cell under test.As described above, computer 112 can obtain this data by way of sourcemeter 110. In step 1808, efficiency (Eff), fill factor (FF), shortcircuit current (Jsc) and open circuit voltage (Voc) are calculated fromthe J-V data. These calculations were described in detail above and thatdescription is incorporated by reference herein.

In step 1810, CNDF 104 is moved to a region of higher light attenuation(e.g., by computer 112 by way of motor 106 and motor control 108). Instep 1812, Jsc-Voc data are obtained from the solar cell under test(e.g., by computer 112 by way of source meter 110). According to anexemplary embodiment, the Jsc-Voc data are obtained concurrently withthe movement of CNDF 104 (i.e., steps 1810 and 1812 are performedconcurrently). In step 1814, pseudo-efficiency (PEff) and pseudo fillfactor (PFF) are calculated from the Jsc-Voc data. These calculationswere described in detail above and that description is incorporated byreference herein.

In step 1816, the light source is turned off (e.g., by closing theshutter on solar simulator 102 rather than turning off the light sourceitself). In step 1818, dark J-V data is obtained from the solar cellunder test. In step 1820, diode ideality factors (n₁ and n₂), reversesaturation current (J₀₁ and J₀₂), shunt resistance (Rshunt) and seriesresistance (Rseries) are calculated from the dark J-V data. Thesecalculations were described in detail above and that description isincorporated by reference herein.

In step 1822, the light source is turned back on again (e.g., by openingthe shutter on solar simulator 102). In step 1824, CNDF 104 is moved toa plurality of regions of lower light attenuation (e.g., by computer 112by way of motor 106 and motor control 108), one of which is atransparent (blank) region. According to an exemplary embodiment, threeregions of different light attenuation are used (e.g., corresponding to100% (by way of the transparent region), 80% and 60% light intensitylevels, respectively). In step 1826, RsX data is obtained from the solarcell under test by obtaining three J-V curves at those three lightintensity levels. In step 1828, series resistance is calculated from theRsX data. This calculation was described in detail above and thatdescription is incorporated by reference herein.

Turning now to FIG. 19, a block diagram is shown of an apparatus 1900for implementing one or more of the methodologies presented herein. Byway of example only, apparatus 1900 can be configured to implement oneor more of the steps of methodology 1800 of FIG. 18 for performing asolar cell electrical characterization, in conjunction with a solar cellcharacterization system, such as system 100 of FIG. 1. As highlightedabove, system 100 includes a computer (e.g., computer 112). Apparatus1900 can be configured to serve as computer 112 in system 100.

Apparatus 1900 comprises a computer system 1910 and removable media1950. Computer system 1910 comprises a processor device 1920, a networkinterface 1925, a memory 1930, a media interface 1935 and an optionaldisplay 1940. Network interface 1925 allows computer system 1910 toconnect to a network, while media interface 1935 allows computer system1910 to interact with media, such as a hard drive or removable media1950.

As is known in the art, the methods and apparatus discussed herein maybe distributed as an article of manufacture that itself comprises amachine-readable medium containing one or more programs which whenexecuted implement embodiments of the present invention. For instance,when apparatus 1900 is configured to implement one or more of the stepsof methodology 1800 the machine-readable medium may contain a programconfigured to open a shutter of a solar simulator to cause light to beemitted from the solar simulator, the solar simulator being part of asolar cell characterization system having the solar simulator and acontinuous neutral density filter having regions of varying lightattenuation levels ranging from transparent to opaque, wherein thecontinuous neutral density filter is located between the solar simulatorand a solar cell under test; position a transparent region of thecontinuous neutral density filter having no light attenuation betweenthe solar simulator and the solar cell under test; obtain light J-V datafrom the solar cell under test; move the continuous neutral densityfilter to position a region of higher light attenuation between thesolar simulator and the solar cell under test; obtain Jsc-Voc data fromthe solar cell under test; close the shutter to prevent light from beingemitted from the solar simulator; obtain dark J-V data from the solarcell under test; open the shutter to cause light to be emitted from thesolar simulator; move the continuous neutral density filter to positiona plurality of regions of lower light attenuation between the solarsimulator and the solar cell under test; and obtain three J-V curvesfrom the solar cell under test to obtain RsX data for the solar cellunder test.

The machine-readable medium may be a recordable medium (e.g., floppydisks, hard drive, optical disks such as removable media 1950, or memorycards) or may be a transmission medium (e.g., a network comprisingfiber-optics, the world-wide web, cables, or a wireless channel usingtime-division multiple access, code-division multiple access, or otherradio-frequency channel). Any medium known or developed that can storeinformation suitable for use with a computer system may be used.

Processor device 1920 can be configured to implement the methods, steps,and functions disclosed herein. The memory 1930 could be distributed orlocal and the processor device 1920 could be distributed or singular.The memory 1930 could be implemented as an electrical, magnetic oroptical memory, or any combination of these or other types of storagedevices. Moreover, the term “memory” should be construed broadly enoughto encompass any information able to be read from, or written to, anaddress in the addressable space accessed by processor device 1920. Withthis definition, information on a network, accessible through networkinterface 1925, is still within memory 1930 because the processor device1920 can retrieve the information from the network. It should be notedthat each distributed processor that makes up processor device 1920generally contains its own addressable memory space. It should also benoted that some or all of computer system 1910 can be incorporated intoan application-specific or general-use integrated circuit.

Optional video display 1940 is any type of video display suitable forinteracting with a human user of apparatus 1900. Generally, videodisplay 1940 is a computer monitor or other similar video display.

Although illustrative embodiments of the present invention have beendescribed herein, it is to be understood that the invention is notlimited to those precise embodiments, and that various other changes andmodifications may be made by one skilled in the art without departingfrom the scope of the invention.

What is claimed is:
 1. A solar testing device, comprising: a solarsimulator; and a continuous neutral density filter in front of the solarsimulator having regions of varying light attenuation levels rangingfrom transparent to opaque, the continuous neutral density filter havingan area sufficiently large to filter all light generated by the solarsimulator, and wherein a position of the continuous neutral densityfilter relative to the solar simulator is variable so as to control alight intensity produced by the device.
 2. The device of claim 1,wherein the solar simulator contains a light source which comprises axenon, halogen or LED lamp.
 3. The device of claim 1, wherein thecontinuous neutral density filter has a linear shape.
 4. The device ofclaim 3, wherein the continuous neutral density filter has an area offrom about 300 cm² to about 1,875 cm².
 5. The device of claim 1, whereinthe continuous neutral density filter has a radial shape.
 6. The deviceof claim 5, wherein the continuous neutral density filter has an area offrom about 350 cm² to about 2,500 cm².
 7. The device of claim 1, furthercomprising a frame to which the continuous neutral density filter ismounted.
 8. The device of claim 1, further comprising a motor configuredto change the position of the continuous neutral density filter relativeto the solar simulator.
 9. A solar cell electrical characterizationsystem for testing a solar cell, comprising: a solar testing devicecomprising a solar simulator and a continuous neutral density filterhaving regions of varying light attenuation levels ranging fromtransparent to opaque, the continuous neutral density filter having anarea sufficiently large to filter all light generated by the solarsimulator; and a solar cell, wherein the continuous neutral densityfilter is located between the solar simulator and the solar cell, andwherein a position of the continuous neutral density filter relative tothe solar simulator and the solar cell is variable so as to control alight intensity falling on the solar cell.
 10. The system of claim 9,wherein the continuous neutral density filter has a linear shape. 11.The system of claim 9, wherein the continuous neutral density filter hasa radial shape.
 12. The system of claim 11, wherein the continuousneutral density filter has a semi-circular shape.
 13. The system ofclaim 9, wherein the solar testing device further comprises: a motorconfigured to change the position of the continuous neutral densityfilter relative to the solar simulator and the solar cell.
 14. Thesystem of claim 13, wherein the continuous neutral density filter has alinear shape and is mounted to a frame, and wherein the solar testingdevice further comprises: a chassis; one or more rollers mounted to thechassis; and a gear train connecting the motor to the one or morerollers, wherein the one or more rollers are in contact with a surfaceof the frame.
 15. The system of claim 13, wherein the continuous neutraldensity filter has a radial shape and is mounted to a frame, and whereinthe motor is attached to the frame.
 16. The system of claim 13, whereinthe solar testing device further comprises: a motor controllerconfigured to provide a drive signal to the motor.
 17. The system ofclaim 9, wherein the solar testing device further comprises: a sourcemeter configured to read electrical data from the solar cell.
 18. Anapparatus for performing a solar cell electrical characterization, theapparatus comprising: a memory; and at least one processor device,coupled to the memory, operative to: open a shutter of a solar simulatorto cause light to be emitted from the solar simulator, the solarsimulator being part of a solar cell characterization system having thesolar simulator and a continuous neutral density filter having regionsof varying light attenuation levels ranging from transparent to opaque,wherein the continuous neutral density filter is located between thesolar simulator and a solar cell under test; position a transparentregion of the continuous neutral density filter having no lightattenuation between the solar simulator and the solar cell under test;obtain light J-V data from the solar cell under test; move thecontinuous neutral density filter to position a region of higher lightattenuation between the solar simulator and the solar cell under test;obtain Jsc-Voc data from the solar cell under test; close the shutter toprevent light from being emitted from the solar simulator; obtain darkJ-V data from the solar cell under test; open the shutter to cause lightto be emitted from the solar simulator; move the continuous neutraldensity filter to position a plurality of regions of lower lightattenuation between the solar simulator and the solar cell under test;and obtain three J-V curves from the solar cell under test to obtain RsXdata for the solar cell under test.